Current limiting fuse for use in rotating machinery



K. w. swAlN Sept. 2, 1969 CURRENT LIMITING FUSE FOR USE IN ROTATNGMACHINERY 2 Sheets-Sheet 1 Original Filed April 13, 1962 Sept. 2, 1969K. w. swA|N 3,465,275

l CURRENT LIMITING FUSE FOR USE IN ROTATING MACHINERY Original FiledApril 13, 1962 2 sheets-sheet 2 Iaweza'orl:

K W'Szhvazw, 'y MJA/ww United States Patent O 3,465,275 CURRENT LIMITINGFUSE FOR USE IN ROTATING MACHINERY Kenneth W. Swain, North Hampton,N.H., assignor to The Chase-Shawmut Company, Newburyport, Mass.

Continuation of application Ser. No. 187,261, Apr. 13,

1962. This application Feb. 26, 1968, Ser. No. 708,028 Int. Cl. H01h 8504 U.S. Cl. 337-158 f 9 Claims ABSTRACT OF THE DISCLOSURE A fusestructure having a fuse link shaped to be braced against centrifugalforces by the pulverulent arc-quenching filler inside the casing of thefuse. The fuse link assembly includes a back-up plate of asynthetic-resinglass-cloth laminate for the fuse link. The fuse linkassembly is designed to minimize let-through currents or let-through 2tvalues, respectively, and to yield fuse strulctures having high currentcarrying capacities in spite of their great compactness. The electricalperformance characteristics of the fuse link assembly are due to thecoaction of a ribbon fuse link having a specific geometricalconfiguration, and its aforementioned back-up plate ofsynthetic-resin-glass-cloth laminate. The back-up plate forms anauxiliary brace for the fragile perforated portion of the fuse link.

This application further discloses a blown fuse indicator having arestraining wire acted upon by braking means and being subject tominimal stresses when the fuse is being rotated.

BACKGROUND OF INVENTION This application is a continuation of myabandoned patent application Ser. No. 187,261, iiled Apr. 13, 1962.

The present invention is an outgrowth of the structure disclosed andclaimed in U.S. Patent 3,020,370 to Kenneth W. Swain, Feb. 6, 1962, forProtection of Semiconductor Diodes, and of the structure disclosed andclaimed in U.S. Patent 3,152,233 to Frederick l. Kozacka, Oct. 6, 1964,for Blade-Type Electric Fuse.

U.S. Patent 3,020,370 discloses a fuse structure intended for use inrotating machinery wherein the fuse link assembly is subjected tointense centrifugal forces. The fuse link assembly disclosed in theabove patent is designed to minimize let-through currents and letthroughi2-t values, respectively, but it does not allow to achieve high currentcarrying capacities without becoming-extremely bulky. An importantfea-ture of the structure disclosed and claimed in U.S. Patent 3,020,370is the provision of fuse link engaging plates ofsyntheticresin-glass-cloth laminate which contribute to the electricalperformance of the fuse and brace the fuse link against the action ofcentrifugal forces. The structure of the above patent is intended forapplications wherein the circuit voltage is relatively low, and whereinthe action of the centrifugal forces is at right angles to thelongitudinal axis of the tubular casing of the fuse.

U.S. Patent 3,152,233 discloses a fuse having a ribbon fuse link of ametal that has a relatively high conductivity and a relatively highfusing point. The fuse link forms a lattice pattern area includingparallel connected necks and serially connected necks. Said latticepattern area is established by substantially transverse lines ofperforations of which each includes a predetermined number ofperforations per line. The number of perforations per line issubstantially in excess of the number of lines of perforations. Theribbon fuse link has a pair of flexible bends adjacent to the axiallyouter ends -thereof establishing portions of the fuse link substantiallyat right angles to the aforementioned lattice pattern area thereof. Aplate of insulating material substantially coextensive with the latticepattern area of the fuse link is aliixed to the fuse link.

The a'bove configuration of the fuse link makes it possible -to controlrelatively high circuit voltages with a relatively small number ofserially connected necks and thus to limit the voltage drop across thefuse when the latter is current carrying. When the fuse blows thecurrent to be interrupted at each point of break is relatively small,due to the presence of a large number of necks of which each forms apoint of break. The insulating plate may evolve arc-quenching blasts ofgas directed against each point of break, and may have the ability toabsorb large amounts of hea-t from each of the parallel arclets andseries arclets formed upon blowing of the fuse.

For the aforementioned reason combining the features of U.S. Patents3,020,370 and 3,152,233 seems to result in a current-limiting fusesuitable for use in rotating machinery, i.e. suitable for back-upprotection of solid state diodes in such machinery. It has, however,been found that neither the fuse link assembly of U.S. Patent 3,152,233,nor any other prior art fuse link assembly, is capable of minimizinglet-through currents and maximizing current-carrying capacity to theextent required for the protection of rotating diodes within the spacelimitations to which such fuses are subject, if the current carryingcapacity of Ithe diodes is relatively high, e.g. in the order of manyhundred amps., and the circuit voltage is relatively high, e.g. in theorder of 800 volts, or even higher.

The aforementioned objects are achieved according to the presentinvention by combining the features ot the structures of U.S. Patents3,020,270 and 3,152,233, and by adding to this combination a meanspredicated on the evolution of an arc-quenching blast of gas whichincreases and prolongs Ithe arc voltage without adversely affecting thecurrent-carrying capacity of the fuse.

The blown fuse indicator according to the present invention is anoutgrowth of prior art flapper type blown fuse indicators shown, forinstance, in a pamphlet by Voigt & Haeffner A.G., Frankfurt am Main,Germany, titled Druckfreie Hochlestungssicherungen, Publication #650(Pressureless High Interrupting Capacity Fuses), published in 1937. Theabove prior art blown fuse indicator is adapted for application in fusesforrotating machinery by reducing the spring bias to which itsrestraining wire is subjected virtually to zero, and by providingbraking means surrounding the restraining wire adjacent one end thereofand acting radially inwardly upon the restraining wire. These brakingmeans absorb a portion of the centrifugal forces to which the flapper orother equivalent blown fuse indicating element is subjected. Thus thecenter region of the restraining wire is normally not subjected to anyspring bias and to but a portion of the centrifugal forces to which thetiapper, or its equivalent, is subjected. When the restraining wiremelts the centrifugal forces acting upon Ithe flapper, or itsequivalent, overcome the braking action of the aforementioned brakingmeans, and thus the flapper, or its equivalent, is moved under theaction of centrifugal forces to the indicating position thereof.

SUMMARY OF INVENTION A current-limiting fuse embodying this inventionincludes a substantially tubular casing of insulating material and apair of terminal plugs having axially inner end surfaces provided withtuse-link-receiving grooves and jointly forming with said casing asolid, substantially cylindrical wall. A pulverulent arc-quenchingfiller is arranged inside the casing. A plate ofsynthetic-resin-glass-cloth laminate is arranged inside the casing,embedded in the arcquenching filler, and has edges braced against saidsolid, substantially cylindrical wall. A fuse link of sheet metalarranged inside the casing, and embedded in the arcquenching filler, hasaxially outer ends projecting into said fuse-link-receiving grooves ofsaid pair of plugs. The fuse link includes a perforated portionextending parallel to, abutting against, and being affixed to said plateof synthetic-resin-glass-cloth laminate, and the fuse link furtherincludes non-perforated portions extending substantially transversely tosaid plate of synthetic-resin-glass-cloth laminate. The perforatedportion of the fuse link has a plurality of transverse lines ofperforations each defining a plurality of spaced parallel-connectednecks. The number of necks per line of perforations exceeds the numberof transverse lines of perforations. The perforated portion of the fuselink further has an additional line of perforations arranged between oneof said pair of terminal plugs and an axially outer of said plurality oftransverse lines of perforations. The number of necks defined by saidadditional line of perforations is smaller than the number of necksdefined by each of said plurality of lines of perforations. Theaggregate width of the necks defined by said additional line ofperforations is smaller than the aggregate width of said plurality ofnecks defined by each of said plurality of lines of perforations.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is in part a top plan view and inpart a section of a fuse structure embodying the invention;

FIG. 2 is a side elevation of the structure of FIG. 1 seen from the leftside thereof;

FIG. 3 is a side elevation of the structure of FIG. 1 seen from theright side thereof;

FIG. 4 is a section taken along 4-4 of FIG. 3 showing the structure ofFIG. 1 on a larger scale;

FIG. 5 is a section taken along 55 of FIG. 3;

FIG. 6 is a bottom plan view of the integral fuse link andfuse-link-support unit forming a part of the structure of FIGS. l and 5;and

FIG. 7 shows diagrammatically the general lay-out of the rotor for anexciter for a brushless electric generator and the way in whichcurrent-limiting fuses embodying this invention are arranged therein.

DESCRIPTION OF PREFERRED EMBODIMENT OF INVENTION The aforementioned U.S.Patent 3,020,370 shows a wiring diagram of the rotor of an exciter for abrushless electric generator including cell fuses adapted to withstandcentrifugal forces of many thousand g (acceleration of gravity), and thecurrent-limiting fuses embodying the present invention are intended tobe included into an electric circuit in the same fashion as shown inFIG. 1 of the above patent.

Referring now to the drawings, numeral 1 has been applied to indicate atubular casing of insulating material, preferably a melamine-glass-clothlaminate, closed on both ends thereof by terminal elements in the formof cylindrical plugs 2 press-fitted into casing 1. Each plug is providedwith a knife blade contact 3 forming an integ'ral part thereof. Bladecontacts 3 are provided with holes 4 for securing the fuse structure tothe rotor of an exciter for a brushless electric generator. Since plugs2 are subjected to high centrifugal forces it is necessary to firmlysecure plugs 2 to casing 1. This is best achieved by radial steel pinsextending radially through casing 1 into radial bores provided in plugs2. These pins have not been shown in the drawings, but FIG. 1 showsclearly radial pin-receiving holes 5 into which the aforementioned pinsare inserted. The axially inner surface 2 of each plug 2 is providedwith four parallel grooves, namely two radially outer grooves 6 and tworadially inner grooves 7. Reference numeral 8 has been applied toindicate a pair of plates of insulating material evolving gas under theheat of electric arcs. Plates 8 may be made, and are preferably made, ofa melamine-glass-cloth laminate. The axially outer ends, or edges, ofplates 8 abut against and are braced by the axially inner end surfaces 2of plugs 2. This is achieved by inserting the aforementioned ends, oredges, of plates 8 into the radially outer grooves 6 and firmly holdingthe same therein by friction. The fuse structure further comprises apair of ribbon fuse links 9, preferably of silver, who-se length exceedsthe spacing between the axially inner end surfaces 2" of terminalelements or terminal plugs 2. The axially outer ends of links 9 areinserted into grooves 7 and the latter are filled with soft solder (notshown). Thus a conductive connection is established between the ends ofeach of links 9 and plugs 2, and plugs 2 are conductively interconnectedby links 9. A portion of each link 9 is bent out of the plane defined byits ends and by the grooves 7 into which its ends are inserted into theplane defined by the insulating plate 8 immediately adjacent thereto. InFIG. 6 reference character 9 has been applied to indicate the ends oflink 9 which are intended for insertion into link-receiving grooves 7,and reference character 9 has been applied to indicate the intermediateportion of link 9 situated between ends 9 and bent into the plane of theimmediately adjacent plate 8. Each of links 9 is provided with aplurality of transverse lines a, b, c, b, c of substantially circularperforations. Each of lines a, b, c, b, c defines a region of reducedcross-section. Lines b of perforations define two regions of reducedcross-section Where the reduction of cross-section is smallest, i.e. thecross-section relatively large. Lines c of perforations define tworegions of reduced cross-section where the reduction of cross-section ishigher than at lines b, i.e. the cross-section at the regions of reducedcross-section defined by the lines of perforation c is less than thecross-section at the regions of reduced cross-Section defined by thelines of perforations b. The line of perforations a has been derivedfrom a line of perforations which was initially identical with the linesof perforations c by integrating each of the two laterally outerperforations of such a line into one single substantially U-shapedperforation of increased size. Line a of perforations defines a regionof reduced cross-section where the reduction of cross-section ishighest, i.e. higher than at lines b and c. In other words, thecross-section at the region of reduced cross-section defined by line ais less than the cross-section at any other point of link 9. Thecross-section at line a being smallest, fusion and arcing will beinitiated at this point of the link on occurrence of fault currents ofsignificant magnitude. The arc voltage at the point of initial fusionand arcing is sufficient to limit momentarily the rate of rise of thefault current, but not sufficient to bring the fault current to zero.While the arc still burns at the region of line a, fusion and arcinitiation occur at the two lines c substantially simultaneously. Thecombined arc voltage formed at the breaks adjacent lines a, c and c issufiicient to rapidly bring the current down to zero. The crosssectionof link 9 at the two points of lines of perforations b is so large thatfusion never occurs at these two points as long as the fuse is usedwithin its rated interrupting capacity. Each transverse line ofperforations b comprises but two perforations. An eyelet 10 whose shankhas a diameter substantially equal to the diameter of the perforationsforming line b is inserted into each of these perforations and projectthrough a registering perforation in one of plates 9. Thus theperforated portion 9 of each fuse link 9 intermediate ends 9 is firmlyclamped against one of plates 9 once the four eyelets 10 which projectthrough each of links 9 and through each of plates 8 are upset.

Each of the two lines c of perforations defines four necks which areconnected in parallel and have the same width. The line of perforationsa defines but two necks which are connected in parallel. The aggregatewidth of the necks of each of lines c is much larger than the aggregatewidth of the necks of line a. As a result, the current density at thenecks of line a is in excess of the current density at lines c. Becauseof the immediate proximity of the necks of line a to one of terminalplugs 2 operating as a heat sink and heat dissipator, the high currentdensity and concomitant heat generation at the two necks of line a doesnot significantly affect, or limit, the currentcarrying capacity of thefuse structure. However, since the two necks of line a fuse first,resulting in a relatively long arcing time at these two points, andsince the current density at the two arclets resulting from melting ofthe two necks is particularly large, the line of perforations a couldnot achieve its intended purpose if its necks were not backed up byplates 8 capable of effectively absorbing heat and of evolving under theheat of arcs highly effective blasts o-f arc-quenching gases.

The right terminal plug 2 as seen in FIGS. l, 4 and 5 is provided withan abutment 2' for transmitting centrifugal forces from the fusestructure to a cooperating abutment member or bracing member integralwith the rotor of an exciter for a brushless electric generator. Thefunction of abutment plate 2' will be explained below more in detail inconnection with FIG. 7. The area of plate 2' exceeds the cross-sectionalarea of casing 1 as determined by the outer diameter thereof. Plate 2 isprovided with a groove 11 which is U-shaped in cross-section. A blownfuse indicator element is arranged within groove 11. This indicatorelement is made of sheet metal and comprises a flat end 12' secured by arivet 18 to plate 2' and this indicator element further comprises asubstantially U-shaped end 12" restrained by fusible wire 13. Sinceindicator element 12', 12" is connected by rivet 18 to plate 2' of rightterminal plug 2 (FIGS. l and 5), and since element 12', 12" is made ofsheet metal, this element is in conductive connection with theaforementioned right plug 2.

Fusible wire 13 forms a shunt across fuse links 9 and interconnectsconductively the two terminal plugs 2. The left end of fusible wire 13,as seen in FIG. 4, extends into a bore in one of plugs 2 and isfrictionally held in position by a resilient cap 14. The right end offusible wire 13, as seen in FIG. 4, is inserted into a bore 15 providedin the right plug 2 and its abutment plate 2 and extends through a finebore in the abutment plate 2' coaxial to bore 15 to the outside of thefuse structure. Bore 15 houses impediments to the flow of products ofarcing to the outside of the fuse structure. These impediments may takevarious forms. As shown in FIGS. 4 and 5 the right end of restrainingwire 13 is in frictional engagement with a block 16 of an elastomer,e.g. wire 13 may be threaded through such a block, thus restraining theright end of wire 13 in position. The indicator element 12', 12" has anormal position and an indicating position. It has been shown in FIGS.3, 4 and 5 in its normal position in which it is normally held byrestraining wire 13. In the indicating position of element 12', 12" theportion 12" thereof is moved out of groove 11 and its flat highlyflexible portion 12' bent. The portion 12" of indicator element 12', 12"which is U-shaped in cross-section is provided with a relatively largeaperture and wire 13 is threaded through said aperture. Wire 13 isfurther threaded through a small hole in a small disc 17 and providedwith a knot at the axially outer side of disc 17. Thus disc 17establishes a conductive connection between wire 13 and indicatorelement 12', 12".

Though the impediments of the outflow of hot produ-cts of arcingarranged in bore 15 are quite effective, safety considerations requirethat the diameter of the passage through which wire 13 projectsoutwardly of the fuse structure, i.e. through plug 2 and plate 2', be asnarrow as possible, i.e. that the clearance between wire 13 and thatpassage be minimized. If indicator 12', 12" were provided with a passagefor wire 13 having -a diameter as small as the passage in parts 2, 2 forwire 13, aligning both passages would involve some difficulties in themanufacture of the device. These difficulties are avoided by providingindicator element 12', 12" with an aperture for the passage of wire 13whose diameter is relatively large and by superimposing upon indicatorelement 12', 12" disc 17, the bore of which through which wire 13 isthreaded is quite small and which is freely movable in the plane ofelement 12', 12" so as to be self-adjusting, i.e. to effect automaticregistration of its narrow passage for wire 13 with the narrow passagefor wire 13 provided in parts 2, 2'.

It will be noted that the two bends of deg. in each fuse link 9 protectthe points of reduced cross-section thereof formed by lines ofperforations a, b, c, b, c against stresses which might result in theabsence of these two bends from 4an accidental change of the spacingbetween the axially inner surfaces 2' of plugs 2.

It will further be noted that the structure shown in the drawings doesnot include any spring means imposing a spring bias upon wire 13; nordoes the structure shown in the drawings include any other auxiliarysource of motive power such as, for instance, a spring for movingindicator element 12', 12" from the normal position shown in thedrawings to the indicating position thereof.

Each terminal plug is provided with two holes for filling a pulverulentarc-quenching ller into casing 1 and each of these holes is closed by aresilient metal cap 14. One of the aforementioned filling holes and capsis arranged under the indicator element 12', 12", as clearly shown inFIG. 3. The filling holes and the caps 14 in both plugs 2 are inalignment, but the filling hole through which wire 13 extends isslightly out of alignment with bore 15 for wire 13. Hence the directionin which wire 13 extends is not strictly parallel to the longitudinalaxis of casing 1 and plugs 2. This slight deviation from parallelism hasbeen indicated in FIG. l in a slightly exaggerated fashion. The clampingaction of cap 14 (see FIG. 4) is not sufficient to establish a goodconductive connection between wire 13 and plug 2 and to firmly hold inposition the left end of wire 13. Hence wire 13 has an end situatedoutside of the left plug 2, as seen in FIGS. 1 and 4, and this end ofwire 13 is screwed against the axially outer surface of said left plug.FIG. 2 shows the end of wire 13 extending out of casing 1 and terminalor clamping screw 19 clamping wire 13 against plug 2.

As clearly shown in FIGS. 1 and 4, insulating plates 8 subdivide casing1 into a radially inner chamber'A and a pair of radially outer chambersB communicating with said radially inner chamber A. Casing 1 is filledwith a pulverulent `arc-quenching filler, preferably quartz sand. Thereis, therefore, a mass of quartz sand in the radially inner chamber A andthere are additional masses of quartzsand in the radially outer chambersB. These masses of arc-quenching filler, or quartz-sand, performdifferent functions in interrupting excessive currents, as will beexplained below more in detail.

Referring now to FIG. 7, numeral 21 has been applied to indicate therotor of an exciter for a brushless electric generator. Rotor 21 ismounted on shaft 22 and supports six semiconductor diodes orsemiconductor rectifier cells 23. Each cell is associated (seriallyconnected) with a cell fuse 24. Each of the cell fuses 24 is a structureof the type shown in FIGS. l-5 and includes a casing 1, terminal plugs2, and an abutment plate 2'. The latter is abutting against an abutment25 integral with rotor 21 tending to support each fuse 24 against thehigh centrifugal forces acting upon it when rotor 21 rotates at high`angular velocities.

It will be apparent from the foregoing that the structure shown in FIGS.1-5 protects the weak points of reduced cross-section of fuse links 9against damage by the action of centrifugal forces. Since the mass ofindicator apper 12', 12" is small the centrifugal forces acting uponwire 13 and tending to stretch wire 13 are relatively limited. Since theright end of wire 13 (FIG. 4) is compressed by elastomeric body 16through which wire 13 is threaded, this arrangement operates as afriction brake, providing further relief to wire 13 from the action ofcentrifugal forces in a direction longitudinally of wire 13. Since thecentrifugal forces upon the fuse struct-ure 24 `are received byabutments 25 the bolts intended to extend through bores 4 in bladecontacts 3 are not subjected to any shearing stresses. Excessivecurrents result in the formation of initial breaks at the lines a ofreduced cross-section of link 9 (FIG. 6). As mentioned above, thearc-voltage generated at these initial points of break is notsufficiently high to force the excess current down to zero, but reducesthe rate of rise of excess currents. Shortly upon formation of initialbreaks at lines a of reduced cross-section additional breaks are formedat the two lines c of reduced cross-section. The aggregate arc voltageformed at all the serially related breaks forces the excess currentrapidly down to zero. The arcs formed at lines a and c are driven intochamber A by the joint action of magnetic forces inherent in thegeometry of the fuse structure and by gases evolving from insulatingplates 8 under the heat of arcs. Some of the mass of quartz-sand inchamber A will fuse and form a fulgurite. The non-fused quartz-sand inchamber A forms a pre-cooler for the products of arcing resulting fromblowing of the fuse. The products of arcing upon having been pre-cooledin chamber A flow into chamber B where the temperature thereof isfurther reduced. In other words, the mass of quartz-sand in chamber Aforms a pre-cooler and the masses of quartz-sand in chambers B formafter-coolers for the products of arcing, and plates 8 form bafliesguiding the flow of products of `arcing inside of casing 1. Wire 13 hasa much higher resistance than ribbon fuse links 9 and, therefore, wire13 does not carry any significant amount of current as long as links 9are intact. Upon fusion of links 9 wire 13 begins to carry current andfuses rapidly. Now indicator element 12', 12 is free to move axiallyoutwardly under the action of centrifugal forces in that direction. Inmoving from its normal position to its indicating position indicatorelement 12', 12 overcomes the friction between the elastomeric brakingbody 16 and the right end of wire 13 (FIG. 4). Inspection of the rotor21 by means of a stroboscopic light source having the proper frequencyreadily reveals whether or not one or more of the fuses 24 carried byrotor 21 have blown.

It will be apparent from the foregoing that I have provided fusestructures for rotating rectifiers that are adapted to be radiallymounted on the rotor in positions like the positions of the spokes of awheel. This makes it possible to provide simple and highly effectivemeans against the destructive action of centrifugal forces and toprovide for simple springless blown fuse indicator means including arestraining wire which is not significantly subjected to stresses andindicating means which are moved only by centrifugal action from thenormal position to the indicating position thereof and which, therefore,do not require any auxiliary source of motive power for that purpose.The mass of the indicator element ought to be sufiiciently large tocause the same to snap instantly to its indicating position upon fusionof the restraining wire 13. Because of the high angular velocities ofrotating machinery this can be achieved with a relatively small mass.

The substantially transverse bends in fuse links 9 have two functions,i.e. they and the pulverulent arc-quenching filler provide jointly aneffective bracing action against centrifugal forces, and furthermore thetransverse bends impart such flexibility to the fuse link assembly 9, 8as necessary to preclude undue stresses in the fragile perforatedportion of the fuse link on account of tolerances and thermal expansionand contraction. The bracing action of the aforementioned transversebends is missing in case that the packing of the pulverulentarc-quenching filler is not sufficiently dense. Then each of plates 8operates as a standby to brace the fuse link 9 with which it isassociated.

The performance of the fuse depends critically upon the geometry of thefuse links 9 which has been described above and the presence and thenature of back-up and bracing plates y8. The lines of perforation a offuse links 9 generate arc voltages prior to the point of time when arcvoltages are generated at the lines of perforations c. Be-

cause of the positioning of the lines of perforations a relative to theadjacent terminal plug 2, the heat generated at the former is largelyabsorbed and dissipated by the latter. Thus the presence of line a ofperforations has but a small limiting effect upon the current-carryingcapacity of the fuse. The tendency of the relatively small arc voltagesgenerated at lines of perforations au to decay rapidly is effectivelycounteracted by plate 8 as described above in detail. These plates havethe dual function of stabilizing the arc voltage at points of highestcurrent density and longest arc duration, and of bracing the fragileperforated portion of the fuse link against the action of centrifugalforces.

It will be understood that I have illustrated and described herein apreferred embodiment of my invention, and that various alterations maybe made therein without departing from the spirit and scope of theappended claims.

I claim as my invention:

1. In a current-limiting fuse structure for use in rotating machinerythe combination of:

(a) a substantially tubular casing of insulating material;

(b) a pair of terminal plugs arranged adjacent to and closing the endsof said casing, said pair of plugs having axially inner end surfacesprovided with fuselink-receiving grooves and jointly forming with saidcasing a solid substantially cylindrical wall;

(c) a pulverulent arc-quenching ller inside said caslng;

(d) a plate of synthetic-resin-glass-cloth laminate arranged inside saidcasing, embedded in said arcquenching filler and having edges bracedagainst said solid substantially cylindrical wall; and

(e) a fuse link of sheet metal arranged inside said casing, embedded insaid arc-quenching filler and having axially outer ends projecting intosaid fuselink-receiving grooves of said pair of plugs, said fuse linkincluding a perforated portion extending parallel to and abuttingagainst and being affixed to said plate of synthetic-resinglass-clothlaminate and said fuse link further including non-perforated portionextending substantially transversely to said plate ofsynthetic-resin-glass-cloth laminate, said perforated portion of saidfuse link having a plurality of transverse lines of perforations eachdefining a plurality of spaced parallel-connected necks, the number ofnecks per transverse line of perforations exceeding the number oftransverse lines of perforations, said perforated portion of said fuselink further having an additional line of perforations arranged betweenone of said pair of terminal plugs and an axially outer of saidplurality of transverse lines of perforations, the number of necksdefined by said additional line of perforations being smaller than thenumber of necks defined by each of said plurality of lines ofperforations, and the aggregate width of said necks defined by saidadditional line of perforations being smaller than the aggregate widthof said plurality of necks defined by each of said plurality of lines ofperforations.

2. A current-limiting fuse structure as defined in claim 1 wherein saidperforations of said plurality of lines of perforations are circular,and wherein said additional line of perforations includes a circularcenter perforation and a pair of substantially U-shaped lateralperforations jointly defining with said center perforation a pair ofparallelconnected necks.

3. A current-limiting fuse structure as specified in claim 1 including(a) a blown fuse indicator element having a normal position and anindicating position and arranged adjacent the outer end surface of oneof said pair of P ugs;

(b) a fusible wire substantially free from any spring bias arrangedinside said casing and conductively connecting said one of said pair ofplugs to the other of said pair of plugs by the intermediary of saidindicator element, said fusible wire -being attached to said indicatorelement and normally restraining said indicator element in said normalposition thereof; and

(c) brake means inside said one of said pair of plugs immediatelyadjacent said indicator element surrounding said fusible wire andexerting a radially inward braking action upon said fusible wire tendingto reduce stresses adjacent the center of said fusible wire resultingfrom centrifugal forces acting upon said indicator element.

4. A current-limiting fuse structure for use in rotating machineryexerting centrifugal forces in a direction longitudinally thereof, saidfuse structure including (a) a substantially tubular casing ofinsulating material;

(b) a pair of terminal plugs arranged adjacent to and closing the endsof said casing, said pair of plugs having grooves on the axially innerend surfaces thereof;

(c) a pulverulent arc-quenching filler inside said cas- (d) a plate ofsynthetic-resin-glass-cloth laminate arranged inside said casingextending in a direction longitudinally thereof, embedded in saidarc-quenching filler and having axially outer ends braced against saidaxially inner end surfaces of said pair of terminal plugs; and

(e) a fuse link of sheet metal arranged inside said casing, embedded insaid arc-quenching filler and having axially outer ends projecting intosaid grooves on said axially inner end surfaces of said pair of plugs,said fuse link including a perforated portion extending parallel to andIabutting against and being affixed to said plate of asynthetic-resin-glass-cloth laminate and said fuse link furtherincluding non perforated portions extending substantially transverselyto said plate of synthetic-resi-n-glass-cloth laminate, said perforatedportion of said fuse link having a plurality of transverse lines ofperforations each defining a plurality of spaced parallel-connectednecks, the number of necks per transverse line of perforations exceedingthe number of transverse lines of perforations, said perforated portionof said fuse link further having an additional line of perforationsarranged between one of said pair of terminal plugs and an axially outerof said plurality of transverse lines of perforations, the number ofnecks defined by said additional line of perforations being smaller thanthe number of necks dened by each of said plurality of lines ofperforations, and the aggregate width of said necks defined by saidadditional line of perforations being smaller than the aggregate widthof said plurality of necks defined by each of said plurality of lines ofperforations.

5. A current-limiting fuse structure as defined in claim 4 wherein saidperforations of said plurality of lines of perforations are circular,and wherein said additional line of perforations includes a circularcenter perforation and a pair of substantially U-shaped lateralperforations jointly defining with said center perforation a pair ofparallel connected necks.

6. A current-limiting fuse structure as specified in claim 4 including(a) a substantially non-spring biased fusible wire arranged inside saidcasing and having one end attached to one of said pair of plugs;

(b) a block of an elastomer surrounding and frictionally engaging theend of said fusible wire remote from said one of said pair of terminalplugs; and

(c) a blown fuse indicator element having a normal position and anindicating position arranged adjacent the outer end surface of the otherof said pair of terminal plugs, the end of said fusible wire remote fromsaid one pair of terminal plugs being attached to said indicator elementand normally restraining said indicator element in said normal positionthereof, and said indicator element having a suiciently large mass to bemoved upon fusion of said fusible wire by centrifugal action against thefrictional action of said block of an elastomer from said normalposition to said indicating position thereof.

7. A current-limiting fuse structure as specified in claim 5 whereinsaid indicator element is adapted to be moved from said normal positionto said indicating position solely by the action of centrifugal forcesacting in a direction substantially longitudinally of said fusible wirewithout the presence of any other source of motive power.

`8. A current-limiting cell fuse for rotating rectiliers comprising:

(a) a tubular casing of insulating material;

(b) a pair of terminal plugs press-fitted into the ends of said casing,one of said pair of plugs having a narrow axial bore therein;

(c) a plate means of insulating material inside said casing directlysupported by said pair of plugs;

(d) a ribbon fuse link inside said casing conductively interconnectingsaid pair of plugs, said link defining a region of reducedcross-sectional area situated between the ends thereof and aflixed tosaid plate means;

(e) a blown fuse indicator element of sheet metal arranged on theaxially outer surface of one of said pair of plugs and having arelatively large aperture therein;

(f) a disc superimposed upon said indicator element having a relativelysmall aperture therein; and

(g) a fusible wire substantially free from any spring bias shunting saidlink and conductively interconnecting said pair of plugs, said wirebeing threaded through said bore in said one of said pair of plugs, saidrelatively large aperture in said indicator element and said relativelysmall aperture in said disc and forming a knot at the axially outersurface of said disc.

9. A current-limiting fuse structure for use in rotating machineryexerting centrifugal forces in a direction longitudinally thereof, saidfuse structure including (a) a substantially tubular casing ofinsulating material;

(b) a pair of terminal plugs arranged adjacent to and closing the endsof said casing, said pair of plugs having grooves on the axially innerend surface thereof;

(c) a body of quartz s-and inside said casing;

(d) a plate of glass-cloth-melamine arranged inside said casing,extending in a direction longitudinally thereof, embedded in said bodyof quartz-sand and having axially outer ends braced against said axiallyinner end surfaces of said pair of terminal plugs;

(e) a fuse link of sheet silver arranged inside said casing, embedded insaid body of quartzsand and having axially outer ends projecting intosaid grooves on said axially inner end surfaces of said pair of terminalplugs, said fuse link including a perforated portion extending parallelto and abutting against and being aixed to said plate ofglass-cloth-melamine and said fuse link further including non-perforatedportions extending substantially transversely to said plate ofglass-cloth-melamine, said perforated portion of said fuse link having aplurality of transverse lines of perforations each defining a pluralityof parallel connected necks, the number of necks per line ofperforations exceeding the number of transverse lines of perforations,said perforated portion of said fuse link further having an additionalline of perforations arranged between one of said pair of terminal plugsand an axially outer of said plurality of transverse lines ofperforations defining a plurality of necks, the number of necks of saidadditional line of perforations being smaller than the number of necksdefined by each of said plurality of lines of perforations, and theaggregate width of said plurality of necks defined by said additionalline of perforations being smaller than the aggregate width of saidplurality of necks dened by each of said plurality element surroundingsaid fusible wire and exerting a radially inward braking action uponsaid -fusible wire tending to reduce stresses adjacent the center ofsaid fusible wire resulting from centrifugal forces upon said indicatorelement.

of lines of perforations;

(f) a blown fuse indicator element having a normal position and anindicating position and arranged adjacent the outer end surface of oneof said pair of References Cited UNITED STATES PATENTS plugs. 102,826,660 3/1958 KOZaCka 337-158 X (g) a fusible wire substantially freefrom any spring acqbs '"1" 337-158 X bias arranged inside said casingand conductively 30804 3 5/1963 Wamket a' 37-1581X connecting said oneof said pair of plugs to the other 32757g1 951966 ggg; a "ggwggyg ofsaidpair of plugs by the intermediary of said indicator element, saidfusible wire being attached to 15 said indicator element and normallyrestraining said v indicator element in said normal position thereof;

and

(h) brake means of an elastomer inside one of said pair of plugsimmediately adjacent said indicator 20 337-159. 227, 295

BERNARD GILHEANY, Primary Examiner H. B. GILSON, Assistant Examiner U.S.C1. X.R.

